Featured paper: Additivated perovskites for more stable solar cells.

"Sandwich" of materials that form the perovskite solar cell developed by the Brazilian team.
“Sandwich” of materials that form the perovskite solar cell developed by the Brazilian team.

Thanks to the contributions of research groups from different countries, perovskite-based solar cells have quickly become competitive in terms of energy conversion efficiency – the percentage of solar energy that is converted into electrical energy – reaching values above 25%. Unfortunately, the good efficiency achieved for these solar cells does not remain throughout their use, mainly because of the instability of their active layer. Composed of materials from the perovskite family, this layer of the sandwich-like solar cell is responsible for absorbing light. Due to moisture, as well as light itself, perovskite degrades and threatens the life cycle of a solar cell.

The problem has been the focus of many researchers in the area, among them, those from the Laboratory of Nanotechnology and Solar Energy (LNES) at Unicamp (Brazil), led by Professor Ana Flávia Nogueira. In recently reported research in the Journal of Materials Chemistry C (impact factor 7.059), LNES members were able to produce more stable perovskite films which allowed manufacturing solar cells with lower efficiency losses over time.

The addition of copolymer P(EO/EP) improved the stability of MAPbI3 perovskite.
The addition of copolymer P(EO/EP) improved the stability of MAPbI3 perovskite.

The strategy adopted was to add to the perovskite a compound that gives it stability without affecting its crystalline structure, from which important properties emerge for solar cell performance. The chosen additive, a copolymer (polymer formed by two different monomers), was added in different concentrations to a solution of lead iodide and methylammonium iodide, which, when crystallized, formed a modified and more stable perovskite film.

The researchers used the spin coating technique to prepare filmes of pure perovskite and “additivated” perovskite. In a material degradation test, the authors exposed the samples to ambient light and humidity for nine days and observed their degradation, which was visible to the naked eye by the yellowing of the films, whose original color is almost black. In the samples with additive, the degradation was delayed by a few days when compared to the pure perovskite samples.

Another test carried out by the team showed the films’ ability to regenerate after an initial degradation caused by exposure to a humidifier. The samples with the additive not only degraded less, but also spontaneously regenerated, almost entirely, thirty seconds after removing the moisture source – a phenomenon known as healing – as can be seen in this video.

“This work demonstrated that incorporating a copolymer based on poly(ethylene oxide) to the perovskite layer can delay and, in some cases, even reverse the degradation process of the film with relation to moisture and lighting,” summarizes Jeann Carlos da Silva, co-author of the article.

box_enTo study in detail the structure and composition of the films, the authors used a series of characterization techniques, including an X-ray diffraction technique (in situ GWAXS), available at the Brazilian National Synchrotron Light Laboratory (LNLS), which allowed to monitor the manufacturing process of the films. Based on the set of characterization results, the authors were able to explain the mechanism that generates the protective effect in perovskite films with additives. According to them, the effect occurs mainly due to the interaction performed by the copolymer, through hydrogen bonds, with the methylammonium cation of the perovskite.  In films without the additive, light and moisture cause part of the methylammonium to shift into the gas state and then leave the perovskite structure, generating the degradation, which is partially irreversible. In the films with the additive, the copolymer retains the methylammonium, which generates films that are more stable and have greater regenerative capacity.

“This study also allowed to investigate the crystallization dynamics of the perovskite containing the copolymer, as well as to understand the formation mechanisms of perovskite/copolymer in humidity and lighting conditions,” highlights Francineide Lopes de Araújo, co-author of the article. “In addition, through characterization techniques such as in situ X-ray diffraction, the study explores an important area in order to understand the material, offering an important contribution to the scientific community and opening new investigation perspectives for the application of polymers in the process of forming and manufacturing perovskite solar cells,” she adds.

Finally, the scientific team manufactured solar cells using perovskite films with and without additives as active layer, and compared their energy conversion efficiency. Initially, the presence of the copolymer decreased the efficiency of the devices, since, as it is an insulating material, it impairs the transfer of electrical charges. However, in the stability tests, when the devices were exposed to humidity and light for twenty days, the perovskite cells with additives performed better.

In numbers: while pure perovskite solar cells started at 17% efficiency and maintained 47% of that value at the end of the test, perovskite devices containing 1.5 mg mL-1% copolymer had an initial efficiency of around 15 %, but retained 68% of efficiency after the 20 days of testing.

“Unfortunately, the problem of stability of perovskite solar cells could not be definitively solved through this research, however, an important way to protect the material was explored, mainly against aggressive exposure to moisture and light, which in the future can be combined with other protection mechanisms,” summarizes Jeann Carlos da Silva. “The research also reinforces the feasibility of incorporating extrinsic compounds to perovskite as protective agents,” he adds.

This study began at LNES in 2016, in the master’s research of Jeann Carlos da Silva, shortly after the development, in that same laboratory, of the first perovskite solar cell prototype in Brazil. The research was completed with the collaboration of the postdoctoral fellow Francineide Lopes de Araújo and other members and former members of the group, always under the guidance of Professor Ana Flávia.

The study was funded by Brazilian agencies FAPESP, CNPq and CAPES, and is the subject of the project “Perovskite Solar Cells for Artificial Photosynthesis” of the Center for Innovation on New Energies (CINE) with support from Shell and Fapesp.

Authors of the paper. From the left: Jeann Carlos da Silva, Francineide Lopes de Araújo, Rodrigo Szostak, Paulo Ernesto Marchezi, Raphael Fernando Moral, Jilian Nei de Freitas and Ana Flávia Nogueira.
Authors of the paper. From the left: Jeann Carlos da Silva, Francineide Lopes de Araújo, Rodrigo Szostak, Paulo Ernesto Marchezi, Raphael Fernando Moral, Jilian Nei de Freitas and Ana Flávia Nogueira.

Featured paper: Analytical contribution to sustainable energy.

[Paper: Influence of charge carriers mobility and lifetime on the performance of bulk heterojunction organic solar cells. D.J. Coutinho, G.C. Faria, D.T. Balogh, R.M. Faria. Solar Energy Materials and Solar Cells, Volume 143, Pages 503-509 (December 2015). DOI:10.1016/j.solmat.2015.07.047]

Analytical contribution to sustainable energy 

A scientific study entirely conducted at the São Carlos Institute of Physics from the University of São Paulo (IFSC-USP) has made significant contribution to the assessment of performance of organic solar cells, devices that are able to produce electricity from sunlight – a renewable, clean, safe and practically inexhaustible source of energy. The results of this piece of research were recently published on the journal Solar Energy Materials & Solar Cells, which has an impact factor of 5.337.

Composition of the bulk-heterojunction organic solar cell used in the experiments reported in the paper. In the active layer, the configuration of the electron acceptor (Blue) and donor (Red) materials.

With a structure comparable to a sandwich, the organic solar cell is comprised of layers of nanometric thickness, made of several materials that execute specific functions in the device.

The so-called “active layer”, the one responsible for the main stages of transforming light (flow of photons) into electric current (flow of electrically charged particles), is made of semiconducting organic materials (whose molecules have carbon atoms).  In the electronic band structure of traditional semiconductors, electrons located in the so-called “valence band” jump from their state when they absorb photons, leaving vacant spaces, or holes, and occupying new places in the so-called “conduction band”. In organic semiconductors, the mechanism that produces the electron-hole pairs is similar, with the difference that, instead of a direct transition from one band to the other, there is a molecular exciton (a system containing one negative charge, and one positive charge), which is easily dissociated, producing free charges (electrons and holes).

For the next stage in the conversion of light into electricity to occur, the active layer of the organic solar cells must have many interface regions between two types of materials: the donor and the acceptor of electrons (usually an electronic polymer and a fullerene derivative, respectively).  If the exciton, in its few picoseconds of existence, manages to reach an interface region, the forces keeping the electron and the hole together are broken, so the donation of the electron from the polymer to the fullerene happens.  At this moment, if no traps are on the way to prevent their movement, electrons and holes flow in opposite directions, attracted and collected by electrode elements, producing an electric current that can be used in an external circuit.

In this succession of stages, efficiency losses in the conversion of solar energy into electrical energy may happen due to several factors. One example is the recombination of electrons and holes after the dissociation of the exciton, which prevents these charge carriers to flow freely. Other examples include defects or impurities in active layer materials, which act as traps for the charge carriers, decreasing their mobility.

The paper published on Solar Energy Materials and Solar Cells reports the results of a series of experiments conducted for the purpose of studying, in detail, the mobility and lifetime of charge carriers (electrons and holes), as a function of temperature, in a bulk-heterojunction organic solar cell produced at IFSC. In this kind of device, the electron donor and acceptor materials coexist in a particular configuration (a nanometric film with a dual-phase structure) that increases the interface area between the two, compared to other possible configurations.

The authors also presented in the paper the results of electric current measurements, based on external applied voltage (J-V) under lighting – one of the most relevant experiments in the characterization of solar cells. In fact, this experiment is necessary for assessing the efficiency of a solar cell.

Organic solar cell during electrical characterization under artificial lighting equivalent to sunlight. In the prototype pictured above, on a 5 x 5 cm plate, five devices are connected in series, producing a total of approximately 2V. The individual efficiency of each device studied is around 4%.

In order to adjust and analyze the experimental results, the authors developed a model, based on a set of equations. This model filled a gap in the scientific literature as, up to its release, these analyses were made from approximations, being inaccurate, or using numerical methods, which require hard time-consuming work.

“To this day, there is no formal description of the J-V curve”, says Roberto Mendonça Faria, full professor at IFSC-USP and corresponding author of the paper. “Our research had the merit of developing a J-V analytical expression, which successfully reproduces the characteristics of an organic solar cell in the event the positive and negative carriers have equal mobility”, he points out. With this expression, he adds, it is possible to carry out a more precise assessment of the cells performance, even for cases in which the electrons and holes do not have the exact same mobility.

Left side: Roberto Mendonça Faria corresponding author of the paper). Right side: Douglas José Coutinho (first author).

The paper also features all the analyses the team managed to do from the experimental results and the model, mainly in regard to some factors leading to efficiency loss in the conversion of light into electricity.

This way, the authors of the paper made a contribution to the challenge of producing sustainable energy. “Energy production is crucial for humanity to keep its economic and social development, but it cannot go on with its terrible side effects, polluting the planet and contributing to global warming”, says Faria.

The results reported in the paper comprise the Master’s and Doctoral studies of Douglas José Coutinho, advised by Professor Faria and financed by Brazilian research funding agencies, FAPESP (São Paulo Research Foundation) and CNPq (National Council for Scientific and Technological Development), including through the CNPq National Institute of Science and Technology for Organic Electronics (INEO).